The present invention relates to calcium pyrophosphate (Ca.sub.2 P.sub.2 O.sub.7) powders, and particularly to the synthesis of the pure gamma phase of calcium pyrophosphate, a starting material for calcium haloapatite fluorescent phosphor host material.
In a typical process for producing calcium haloapatite phosphor, dicalcium orthophosphate (CaHPO.sub.4) powder is blended with other components (e.g., manganese carbonate (MnCO.sub.3), calcium chloride (CaCl.sub.2), calcium fluoride (CaF.sub.2), antimony oxide (Sb.sub.2 O.sub.3), or calcium carbonate (CaCO.sub.3)) and fired at high temperature to form the phosphor. During firing, water is driven off from the dicalcium orthophosphate by the high reaction temperature. The quantity of powder which may be fired in each reaction vessel thus is limited by the need to accommodate the resulting water vapor within the reaction vessel.
Attempts have been made to produce calcium haloapatite phosphors by converting the dicalcium orthophosphate to calcium pyrophosphate before blending the phosphate powder with the other starting components. However, three polymorphs of calcium pyrophosphate exist, the alpha, beta, and gamma forms. The gamma phase is preferred for calcium halophosphate phosphor synthesis, because the phosphate is already in the anhydrous form and will not undergo dehydration in the reaction vessel. The beta phase can inhibit the reaction converting the calcium pyrophosphate to the desired calcium haloapatite phosphor host.
Prior to the present invention, no process had resulted in pure gamma phase calcium pyrophosphate, to provide a more desirable intermediate for the production of calcium haloapatite phosphor host. The process described herein was developed to address that need.